Extend front matter support

[alexmozaidze]

Feature

Allow specifying language to use in front matter like so:

---toml
answer = 42
---

Reason

11ty uses gray-matter for parsing front matter, which supports using any language as front matter, but doesn't support +++ notation for TOML.

+++ support workaround for gray-matter

In order to hack in support for +++, you have to make +++ a delimiter, and make TOML the default engine. Here's an example with 11ty config:

const toml = require("@iarna/toml");
module.exports = function(cfg) {
  cfg.setFrontMatterParsingOptions({
    engines: { toml: toml.parse.bind(toml) },
    language: "toml",
    delimiters: "+++",
  });
};